Process for the preparation of 2,6-dimethylnaphthalene

ABSTRACT

A highly selective process is described for preparing 2,6-dimethylnaphthalene which comprises reacting a naphthalene hydrocarbon selected from naphthalene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, polymethylnaphthalenes or their mixtures with one or more benzene hydrocarbons selected from benzene, toluene, xylenes, trimethylbenzenes, tetramethylbenzenes, pentamethylbenzene and/or hexamethylbenzene, under at least partially liquid phase conditions, in the presence of a zeolite belonging to the structural type MTW and optionally in the presence of a methylating agent.

This is a Division of application Ser. No. 09/281,961, filed Mar. 31,1999 now U.S. Pat. No. 6,147,270.

A highly selective process is described for preparing2,6-dimethylnaphthalene which comprises reacting a naphthalenehydrocarbon selected from naphthalene, methylnaphthalenes,dimethylnaphthalenes, trimethylnaphthalenes, polymethylnaphthalenes ortheir mixtures with one or more benzene hydrocarbons selected frombenzene, toluene, xylenes, trimethylbenzenes, tetramethylbenzenes,pentamethylbenzene and/or hexamethylbenzene, under at least partiallyliquid phase conditions, in the presence of a zeolite belonging to thestructural type MTW and optionally in the presence of a methylatingagent.

2,6-dimethylnaphthalene is an intermediate in the synthesis of2,6-naphthalenedicarboxylic acid, used as monomer in the preparation ofPEN (polyethylnaphthalate). It is known that it can be recovered fromfractions coming from the reforming of kerosine (U.S. Pat. No.4,963,248) or from fractions of FCC oil (European Chemical News, page30, Sep. 28, 1992). In the first case, the dimethylnaphthalenes must beseparated by distillation, then, the 2,6 isomer is isolated by means ofselective absorptions and/or crystallization. In the second case, thereis an additional problem due to the presence of nitrogen and sulfurwhich poison the catalysts used for the separation and/or isomerizationphases. There is also a process (U.S. Pat. No. 4,990,717; U.S. Pat. No.5,118,892; U.S. Pat. Nos. 5,073,670; 5,030,781; 5,012,024) which, bymeans of alkenylation, cyclization, dehydrogenation, isomerization,leads to the selective synthesis of 2,6-dimethylnaphthalene. This highnumber of passages obviously represents a burden from an economicalpoint of view and in addition, each passage or chemical reactioninvolves secondary reactions requiring separations to guarantee thepurity of the intermediates or end-product. U.S. Pat. No. 5,043,501describes a synthesis method of 2,6-dimethylnaphthalene in two steps.The first comprises the alkylation of an alkylaromatic with a C₅ olefinin the presence of a zeolitic catalyst, the second comprises adehydrocyclization at 400-500° C. with a catalyst consisting of Pt/Ba/Kon zeolite L, obtaining a product containing dimethylnaphthalenes whichare then isomerized mainly to 2,6 isomer.

S. B. Pu and T. Inui describe in Applied Catalysis A, General 146 (1996)305-316, the alkylation with methanol of methylnaphthalene, catalyzed byzeolites of the BEA, FAU and MTW groups, carried out without a solventand in an exclusively gaseous phase. The best results are obtained withbeta zeolite and faujasite. A. S. Loktev and P. S. Chekriy, in Zeolitesand Related Micro-porous Materials: State of art 1994, SSSC vol. 84, J.Weitkamp et al. (Eds) describe the alkylation of naphthalene ormethylnaphthalene with methanol catalyzed by ZSM-12, carried out in thepresence of paraffinic solvents and in gaseous phase. The yields todimethylnaphthalenes, and to 2,6 isomer in particular, are zero or inany case negligible. In addition, high quantities of heavy by-productsare obtained, due to the reaction conditions and/or decomposition of theparaffinic solvents used.

U.S. Pat. No. 4,795,847 discloses a process for the preparation of2,6-dialkylnaphthalenes which comprises the alkylation in gaseous phaseof naphthalene or 2-alkyl-naphthalene with an alkylating agent in thepresence of a zeolite selected from mordenite, EU-1, offretite, ZSM-12,ZSM-5 and ZSM-22. In the case of the methylation of naphthalene or2-methyl-naphthalene the use of ZSM-5 is particularly preferred. Toreduce undesired isomerization reactions which result in the formationof 1-alkyl-naphthalene, the zeolitic catalyst is previously subjected toprecarbonization treatment. An example is provided of the alkylation of2-methylnaphthalene with methanol, in gas phase, catalyzed by ZSM-5: theconversion obtained between 0.5 and 8 hours is from 5 to 7%, the yieldto dimethylnaphthalenes ranges from 4 to 5% and the2,6-dimethylnaphthalene isomer forms 50% of the dimethylnaphthalenefraction.

We have now unexpectedly found a process for preparing2,6-dimethylnaphthalene starting from naphthalene, methylnaphthalenes,dimethylnaphthalenes, trimethylnaphthalenes and/orpolymethylnaphthalenes carried out under at least partially liquid phaseconditions and in the presence of suitable aromatic hydrocarbons,catalyzed by a zeolite of the structural type MTW (abbreviation IZA),which allows better results to be obtained in terms of yield,selectivity, conversion to useful products in the time unit and life ofthe catalyst, with respect to what is described in the prior art. Inparticular MTW zeolites, when used under the conditions of ourinvention, are more active not only than the same zeolites used underthe conditions described previously, but also with respect to the BEAand MFI zeolites already described in the prior art as the bestcatalysts for the preparation of 2, 6 -dimethylnaphthalene.

The present invention therefore relates to a process for preparing2,6-dimethylnaphthalene which comprises reacting a naphthalenehydrocarbon selected from naphthalene, methylnaphthalenes,dimethylnaphthalenes, trimethylnaphthalenes, pentamethylnaphthalenes,hexamethylnaphthalenes or their mixtures with one or more benzenehydrocarbons selected from benzene, toluene, xylenes, trimethylbenzenes,tetramethylbenzenes, pentamethylbenzene and/or hexamethylbenzene, underat least partially liquid phase conditions, in the presence of a zeolitebelonging to the structural type MTW and optionally in the presence of amethylating agent.

The naphthalene hydrocarbon is preferably selected from naphthalene,methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, ortheir mixtures. A particularly preferred aspect is that the reagent isnaphthalene and/or methylnaphthalene, optionally mixed withdimethylnaphthalenes and/or trimethylnaphthalenes.

Fractions containing naphthalene, methylnaphthalenes anddimethylnaphthalenes, obtained by distillation from FOC (Fuel OilCracking) or LCO (Light Cycle Oil) cracking oils or obtained by thedistillation of distilled oil from pit-coal tar, can be used directly inthe process of the present invention.

According to a particularly preferred aspect of the present invention,the process for the preparation of 2,6-dimethylnaphthalene is carriedout in the presence of the methylating agent which can be selected frommethanol, dimethylether, dimethylcarbonate, dimethylsulfate, methyliodide. Methanol is preferably used.

Operating according to our invention, a high selectivity is unexpectedlyobtained, which, with respect to thermodynamic equilibrium values, isunbalanced towards the 2,6 isomer: there are consequently kineticcontrol conditions of the reaction. The molar ratio between the 2,6 and2,7 isomers of dimethynaphthalene can also be considered as selectivityindex reached with our invention. Whereas the thermodynamic ratio isabout 1 (S. B. Pu and T. Inui, in Applied Catalysis A, General 146(1996) 305-316) in the case of zeolitic catalysts with an MFI and BEAstructure it is 1 and 1.3 respectively, in the case of MTW zeolites usedin gas phase it is 1.2 and 1.5, in the case of MTW zeolites usedaccording to our invention, this ratio varies from 1.9 to 2.7.

For evaluating the selectivity which can be reached with our invention,the selectivity to 2,6 or 2,6+1,6 isomer can also be considered.According to the thermodynamic equilibrium (calculated by S. B. Pu andT. Inui, in Applied Catalysis A, General 146 (1996) 305-316) there is12% of the 2,6 isomer and 14% of the 1,6 isomer. With the process of ourinvention, values are registered equal to 26-35% for the 2,6 isomer and25-32% for the 1,6 isomer, depending on the reaction conditions: in anycase the values are higher than the thermodynamic ones.

Operating under the conditions of our invention, and in the presence ofone or more of the particular benzene hydrocarbons selected, zeolites ofthe structural type MTW give better results than those of zeolites ofthe MFI group, not only because they are more active, with a longer lifeand therefore greater productivity, but because they provide a productcontaining a higher percentage of 1,6 and 1,5 isomers which can beeasily converted to 2,6 isomer by means of commercial processes or knownmethods such as those for example described in EP 519165 and U.S. Pat.No. 5,012,024. In addition, the MFI zeolites produce significantquantities, which tend to increase with deactivation, ofethylnaphthalene which on the other hand is not revealed by GC analysisamong the products obtained according to the process of our invention.With respect to BEA zeolites, the MTW zeolites used according to ourinvention are more selective, more active, with a longer life andproduce less tri and tetra-methylnaphthalenes (with the sameconversion).

Zeolites of the MTW structural type which can be used in the presentinvention are for example: ZSM-12, CZH-5, Nu-13, Theta-3 and TPZ-12.

The zeolite CZH-5 is described in GB 2079735A; Nu-13 is described inEP59059; Theta-3 is described in EP 162719 and TPZ-12 in U.S. Pat. No.4,557,919.

The zeolite of the MTW structural type which is best suited for use inthe present invention is a silico-aluminate with a molar ratioSiO₂/Al₂O₃ greater than or equal to 20. This zeolite and its preparationare described in A.Katovic and G.Giordano, Chem. Ind. (Dekker)(Synthesis of Porous Materials) 1997 69, 127-137. The aluminum can betotal or partly substituted by B, Ga, Fe or their mixtures as describedby Toktarev & Ione, in Chon et al., Progress in Zeolites and MicroporousMaterials, SSSC, vol. 105, 1997.

According to a preferred aspect the zeolite ZSM-12 is used, a porouscrystalline material having in its calcined and anhydrous form a molarcomposition of oxides corresponding to the following formula:

1.0±0.4M_(2/n)O·W₂O₃·20-500YO₂·zH₂O

wherein M is H⁺ and/or a cation of an alkaline or earth alkaline metalwith valence n, W is selected from aluminum, gallium or their mixtures,Y is selected from silicon or germanium, z is between 0 and 60. M ispreferably selected from sodium, potassium, hydrogen or their mixtures.W is preferably aluminum and Y is preferably silicon. W can be at leastpartially substituted by boron, iron or their mixtures. The zeoliteZSM-12 is described in U.S. Pat. No. 3,832,449, in Ernst et al.,Zeolites, 1987, Vol. 7, September, and in Toktarev & Ione, Chon et al.,Progress in Zeolites and Microporous Materials, SSSC, vol. 105, 1997.

A particularly preferred aspect of the present invention is that thezeolite of the MTW type used is in the form in which the cation sitespresent in its structure are occupied for at least 50% by hydrogen ions.It is particularly preferable for at least 90% of the cation sites to beoccupied by hydrogen ions.

The zeolite can be used as such, pelletized in pure form, or extrudedwith suitable inorganic oxide ligands to form cylindrical, sphericalpellets, or with other forms commonly used, or obtained as microspheresby spray-drying after mixing with a ligand. The ligands can be forexample, aluminas, silicas, silicoaluminas, titania, zirconia or clays.Alumina is preferably used. In the bound catalyst the zeolite and theligand are in a weight ratio ranging from 10:90 to 90:10, preferablyfrom 25:75 to 75:25.

The benzene hydrocarbon is selected from benzene, toluene, xylenes,trimethylbenzenes, tetramethylbenzenes, pentamethylbenzene,hexamethylbenzene or their mixtures, and is preferably trimethylbenzene.It is obviously necessary to operate in the presence of the methylatingagent when only benzene is used as benzene hydrocarbon together withnaphthalene alone as naphthalene substrate. The process of the presentinvention is completely different from the known simple alkylationprocesses of the prior art: in fact it is the result of contemporaneoustransalkylation, deproportioning, isomerization and alkylation reactionsin which all the types of methyl present in the reaction mixtureunexpectedly participate, directly or indirectly, in the methylation ofthe naphthalene substrate used and contribute to obtaining very highselectivities. Methyls present in the reaction mixture refer to boththose deriving from benzene hydrocarbon and also those possibly alreadypresent on one or more of the naphthalene substrates used.

In accordance with this, at the end of the process, the benzenehydrocarbon or mixture of benzene hydrocarbons used will produce amixture of corresponding benzene hydrocarbons differently methylatedboth quantitatively and qualitatively, arising from the processesmentioned above. The best results are obtained when a methylating agentis used and consequently at least part of the methyls which are found inthe 2,6-dimethylnaphthalene end-product derive directly or indirectlyfrom this, i.e. by alkylation on the part of the methylating agent ofthe aromatic hydrocarbon and subsequent transalkylation on thenaphthalene substrate.

The feeding of the benzene hydrocarbon is such as to obtain a molarratio between the hydrocarbon and naphthalene groups ranging from 1 to100, more preferably from 3 to 20, naphthalene groups referring to thenaphthalene hydrocarbon used as substrate or, when several naphthalenehydrocarbons are present, the sum of their moles.

When the process of the present invention is carried out in the presenceof a methylating agent, preferably methanol, a molar ratio betweenmethylating agent and the naphthalene groups of less than 30 is used,preferably ranging from 0.1 to 3.

The reaction temperature ranges from 200° C. to 450° C., preferably from250 to 390° C., even more preferably from 280 to 350° C., the WHSV spacevelocity ranges from 0.01 to 8 hours⁻¹, preferably from 0.05 to 1hours⁻¹.

It should be pointed out that the combination of temperature andpressure conditions used should be such as to guarantee that thesynthesis of the 2,6-dimethylnaphthalene at least partly takes place inliquid phase, and even more preferably substantially in liquid phase.

The pressure used can vary from 3 to 60 atm.

The process of the present invention can be industrially carried out incontinuous, semicontinuous or batch; in order to keep the temperaturewithin a preferred range, the catalyst can be arranged in the reactor invarious layers. A quench with naphthalene, with the hydrocarbon ormixture of benzene hydrocarbons used in the process itself, or with themethylating agent, preferably methanol, when present, can be carried outbetween one layer and another.

The temperature control can be carried out not only by means of a quenchof the reagents and/or inert products, but also by intercooling betweenthe layers, for example by means of the interpositioning of coolants.The synthesis of 2,6-dimethylnaphthalene can be conveniently carried outeither in a reactor in which the catalyst is arranged in two or morebeds or in two or more reactors in series, intercooled to control thetemperature.

When an alkylating agent is used, it can be fed in two or more steps.The alkylating agent is preferably fed in two or more steps along thecatalytic beds of the reactor or between these, and/or between thereactors situated in series.

According to a preferred aspect, in order to maximize the production of2,6-dimethylnaphthalene, the product obtained according to the processof the present invention can be separated into:

(a) a fraction containing benzene hydrocarbons, naphthalene andmethylnaphthalene, (b) a fraction containing dimethylnaphthalenes and(c) a fraction containing polymethylated naphthalenes. The desired2,6-dimethylnaphthalene isomer is isolated from the fraction (b) ofdimethylnaphthalenes, whereas the remaining fraction (d) containingdimethylnaphthalenes different from the 2,6 isomer, and fractions a) andc), are re-fed to the initial reactor where they re-enter the reactivecycle. Alternatively, fraction (d) and fractions (a) and (c), optionallyenriched with naphthalene and/or methylnaphthalene, can be fed to aspecific reactor where they are reacted, under at least partially liquidphase conditions, in the presence of a zeolite belonging to the MTWstructural type, with one or more benzene hydrocarbons selected frombenzene, toluene, xylenes, trimethylbenzenes, tetramethylbenzenes,pentamethylbenzenes and/or hexamethylbenzene. The reaction temperatureranges from 200 to 450° C., and the space velocity ranges from 0.01 to 8hours⁻¹.

According to another aspect of the present invention, to maximize theproduction of 2,6-methylnaphthalene, fraction (d) containingdimethylnaphthalenes different from 2,6-dimethylnaphthalene, inparticular the 1,6 and 1,5 isomer, is subjected to isomerization, underat least partially liquid phase conditions, in the presence of acatalyst containing an MTW zeolite, at a temperature ranging from 100 to400° C., more preferably from 120 to 250° C., even more preferably from130 to 200° C.

This particular isomerization process of 1,6-dimethylnaphthalene and1,5-dimethylnaphthalene, pure or mixed with other dimethylnaphthaleneisomers, to give 2,6-dimethylnaphthalene, catalyzed by a zeolite of theMTW type, is in itself innovative and represents a further object of thepresent invention.

The exhausted catalyst deriving from the process for preparing2,6-dimethylnaphthalene can be regenerated with the known combustionmethods of coke or of its precursors to which the deactivation of thesolid acid materials which catalyze reactions involving hydrocarbons, isdue. We have also unexpectedly found a method for regenerating thisexhaused catalyst, which is much simpler and more economical. This newmethod does not require, with respect to the traditional regenerativemethods, either removal of the catalyst from the reaction environment orthe high temperatures necessary for the combustion of coke. A furtherobject of the present invention therefore relates to a method forregenerating the exhausted catalyst deriving from the process for thepreparation of 2,6-dimethylnaphthalene by the reaction of a naphthalenehydrocarbon selected from naphthalene, methylnaphthalenes,dimethylnaphthalenes, trimethylnaphthalenes, tetramethylnaphthalenes,pentamethylnaphthalenes, hexamethylnaphthalene or their mixtures withone or more benzene hydrocarbons selected from benzene, toluene,xylenes, trimethylbenzenes, tetramethylbenzenes, pentamethylbenzeneand/or hexamethylbenzene, under at least partially liquid phaseconditions, in the presence of a zeolite belonging to the MTW structuraltype and optionally in the presence of a methylating agent, wherein saidregeneration method comprises treating the exhausted catalyst with oneor more of these benzene hydrocarbons, at a temperature ranging from 200to 450° C., more preferably from 250 to 400° C., even more preferablyfrom 280 to 370° C., said temperature being at least equal to that usedduring the preparation process of 2,6-dimethylnaphthalene from which theexhausted catalyst derives. The regeneration conditions are selected soas to operate in at least partially liquid phase, the WHSV spacevelocity can range from 0.01 to 8 hours⁻¹ and the pressure can beselected from 5 to 60 atm.

EXAMPLE 1 Preparation of the Catalyst ZSM-12

2.4 grams of sodium aluminate at 56% of Al₂O₃ are dissolved in 84 gramsof aqueous solution of tetraethylammonium hydroxide at 35%. The limpidsolution thus obtained is poured, under stirring, into 200 grams ofcolloidal silica Ludox HS 40. After brief stirring a limpid andhomogeneous gel is obtained which is poured into an AISI316 steelautoclave equipped with an anchor stirrer. The gel is left tocrystallize under hydrothermal conditions at 165° C. for 90 hours.

At this point the autoclave is cooled and the solid separated from themother liquor and washed with demineralized water until the washingwater has a pH of less than 9.

The solid is calcined at 550° C. in an atmosphere of air for 5 hours. Itis then suspended in a solution of demineralized water and ammoniumacetate, the latter in a molar quantity in excess, in particular 5times, with respect to the aluminum formally present from the synthesis.During this operation the sodium contained from the synthesis in thezeolite is substituted with the ammonium ion by ion exchange effect.This first exchange is followed by a washing, a second exchange usingthe same procedure and another washing. The solid is then definitelyseparated from the aqueous environment, dried and calcined for 5 hoursat 550° C. in an atmosphere of air. The zeolitic catalyst is thusobtained in acid form. XRD analysis is carried out on the end-samplewhich shows the presence of only zeolitic crystalline phase of the MTWtype, together with a chemical analysis on the basis of which theresidual sodium content proves to be less than 50 ppm and the molarratio SiO₂/Al₂O₃ is 100.

EXAMPLE 2

2 gr of ZSM-12 zeolite of example 1 (molar ratio SiO₂/Al₂O₃=100)pelleted and granulated within a range of 20-40 mesh, are charged intothe isothermal zone of a fixed bed reactor, with quartz above and belowas inert filling. The reactor is heated to 200° C. for at least 2 hours,under a stream of nitrogen against atmospheric pressure. Maintainingunder a stream of inert gas, the reactor is cooled to room temperature,and the reagents are then fed until the reactor is pressurized to 40bars.

The mixture of reagents fed consists of 1,2,4-trimethylbenzene,naphthalene, methanol in the following molar ratios:1,2,4-trimethylbenzene/naphthalene=10, methanol/naphthalene=3. At thispoint the mixture is heated and brought to a temperature of 350° C. Inthis test the conditions are therefore liquid phase, with respect to thestate of reagents and products. The WHSV (hours⁻¹) with respect to thetotal mixture is 0.86.

The products leaving the reactor are cooled and analyzed bygaschromatography. Samples are taken of the products at regular timeintervals (time on stream). The conversion of the methanol is alwaystotal.

a) The conversion of naphthalene after 28.2 hours is equal to 85.3%

The selectivities of each product with respect to the naphthalene,defined as ratio between the moles of said product formed and the molesof naphthalene converted, are:

selectivity dimethylnaphthalenes (% moles): 54.3

selectivity 2,6-dimethylnaphthalene (% moles): 16.0

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 33.4

selectivity methylnaphthalenes (% moles): 34.6

selectivity polymethylnaphthalenes (% moles): 11.2

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 29.4 (the thermodynamic isomerical distribution, as described inS. B. Pu and T. Inui, Applied Catalysis A, General 146 (1996) 305-316,foresees 12.0% of 2,6 isomer).

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 61.5 (thermodynamic ratio 32%)

molar ratio 2,6/2,7 dimethylnaphthalene: 1.9 (thermodynamic ratio 1)

b) The conversion of naphthalene after 95.5 hours is equal to 50.8%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 32.5

selectivity 2,6-dimethylnaphthalene (% moles): 9.0

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 18.6

selectivity methylnaphthalenes (% moles): 64.6

selectivity polymethylnaphthalenes (% moles): 2.9

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 27.5

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 57.2

molar ratio 2,6/2,7 dimethylnaphthalene: 2.0

c) The test is then carried out halving the space velocity. Theconversion of naphthalene after 102 hours is equal to 59.4%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 38.2

selectivity 2,6-dimethylnaphthalene (% moles): 10.8

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 22.5

selectivity methylnaphthalenes (% moles): 56.7

selectivity polymethylnaphthalenes (% moles): 5.1

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 28.3

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 59.0

molar ratio 2,6/2,7 dimethylnaphthalene: 2.0

EXAMPLE 3

4 gr of ZSM-12 zeolite (molar ratio SiO₂/Al₂O₃=100), prepared inaccordance with example 1, pelleted and granulated within a range of20-40 mesh, are charged into the isothermal zone of a fixed bed reactor,with quartz above and below as inert filler. The reactor is heated to200° C. for at least 2 hours, under a stream of nitrogen againstatmospheric pressure.

Maintaining under a stream of inert gas, the reactor is cooled to roomtemperature, and the reagents are then fed until the reactor ispressurized to 35 bars.

The mixture of reagents fed consists of 1,2,4-trimethylbenzene,naphthalene, methanol in the following molar ratios:1,2,4-trimethylbenzene/naphthalene=18, methanol/naphthalene=2. At thispoint the mixture is heated and brought to a temperature of 320° C. Inthis test the conditions are therefore liquid phase, with respect to thestate of reagents and products. The WHSV (hours⁻¹) with respect to thetotal mixture is 0.43.

The products leaving the reactor are cooled and analyzed bygaschromatography. Samples are taken of the products at regular timeintervals (time on stream). The conversion of the methanol is alwaystotal.

a) The conversion of naphthalene after 91 hours is equal to 66.5%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 41.2

selectivity 2,6-dimethylnaphthalene (% moles): 13.0

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 26.8

selectivity methylnaphthalenes (% moles): 53.7

selectivity polymethylnaphthalenes (% moles): 5.1

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 31.5 (the thermodynamic isomeric distribution foresees 12.0% of2,6 isomer)

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 65.1 (thermodynamic ratio 32%)

molar ratio 2,6/2,7 dimethylnaphthalene: 2.1 (thermodynamic 1)

b) The conversion of naphthalene after 241 hours is equal to 59.1%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 36.8

selectivity 2,6-dimethylnaphthalene (% moles): 11.2

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 23.3

selectivity methylnaphthalenes (% moles): 58.9

selectivity polymethylnaphthalenes (% moles): 4.3

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 30.5

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 63.3

molar ratio 2,6/2,7 dimethylnaphthalene: 2.3

c) At this point, in order to regenerate the catalyst only1,2,4-trimethylbenzene is sent to the reactor at a WHSV of 1.32 hours⁻¹and a temperature of 360° C. (under liquid phase conditions, pressure of35 bars) for 24 hours. The temperature is then brought to 320° C. andthe initial mixture with naphthalene and methanol is fed again. After310 hours of running (of which 286 hours under a stream also containingnaphthalene) a sample is taken from the reactor and analyzed, asdescribed above. The conversion of naphthalene after 310 hours is equalto 70.9%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 45.7

selectivity 2,6-dimethylnaphthalene (% moles): 14.4

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 29.9

selectivity methylnaphthalenes (% moles): 47.9

selectivity polymethylnaphthalenes (% moles): 6.4

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 31.5

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 65.4

molar ratio 2,6/2,7 dimethylnaphthalene: 2.3

The washing with only benzene hydrocarbon unexpectedly allowed recoveryof the catalytic activity, without resorting to the methods normallyused in the field of the combustion catalysis of coke or its precursors,which is typically responsible for the deactivation of a solid acidwhich catalyzes reactions involving hydrocarbons.

EXAMPLE 4

4 gr of ZSM-12 zeolite (molar ratio SiO₂/Al₂O₃=100), prepared as inexample 1, pelleted and granulated within a range of 20-40 mesh, arecharged into the isothermal zone of a fixed bed reactor, with quartzabove and below as inert filling. The reactor is heated to 200° C. forat least 2 hours, under a stream of nitrogen against atmosphericpressure. Maintaining under a stream of inert gas, the reactor is cooledto room temperature, and the reagents are then fed until the reactor ispressurized to 35 bars. The mixture of reagents fed consists of1,2,4-trimethylbenzene, naphthalene, methanol in the following molarratios: 1,2,4-trimethylbenzene/naphthalene=18, methanol/naphthalene=2.At this point the mixture is heated and brought to a temperature of 300°C. In this test the conditions are therefore liquid phase, with respectto the state of reagents and products. The WHSV (hours⁻¹) with respectto the total mixture is 0.43.

The products leaving the reactor are cooled and analyzed bygaschromatography. Samples are taken of the products at regular timeintervals (time on stream).

The conversion of the methanol is always total.

a) The conversion of naphthalene after 147 hours is equal to 46.6%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 30.6

selectivity 2,6-dimethylnaphthalene (% moles): 9.7

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 19.8

selectivity methylnaphthalenes (% moles): 69.4

selectivity polymethylnaphthalenes (% moles): 0.0

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 31.7 (the thermodynamic isomeric distribution foresees 12.0% of2,6 isomer)

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 64.8 (thermodynamic ratio 32%)

molar ratio 2,6/2,7 dimethylnaphthalene: 2.5 (thermodynamic 1)

b) The conversion of naphthalene after 506 hours is equal to 28.2%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 17.9

selectivity 2,6-dimethylnaphthalene (% moles): 6.1

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 11.7

selectivity methylnaphthalenes (% moles): 80.0

selectivity polymethylnaphthalenes (% moles): 2.0

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 34.2

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 65.4

molar ratio 2,6/2,7 dimethylnaphthalene: 2.8

c) At this point, in order to regenerate the catalyst only1,2,4-trimethylbenzene is sent to the reactor at a WHSV of 1.32 hours⁻¹and a temperature of 320° C. (under liquid phase conditions, pressure of35 bars) for 60 hours. The temperature is then brought to 300° C. andthe initial mixture with naphthalene and methanol is fed again. After600 hours of running (of which 540 hours under a stream also containingnaphthalene) a sample is taken from the reactor and analyzed, asdescribed above. The conversion of naphthalene after 600 hours is equalto 43.6%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 32.4

selectivity 2,6-dimethylnaphthalene (% moles): 9.7

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 20.8

selectivity methylnaphthalenes (% moles): 67.6

selectivity polymethylnaphthalenes (% moles): 0.0

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 30.0

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 64.2

molar ratio 2,6/2,7 dimethylnaphthalene: 2.5

Also in this case, washing with only benzene hydrocarbon unexpectedlyallowed recovery of the catalytic activity, without resorting to themethods normally used in the field of the combustion catalysis of cokeor its precursors, which is typically responsible for the deactivationof a solid acid which catalyzes reactions involving hydrocarbons.

EXAMPLE 5 (comparative)

2 gr of beta zeolite (molar ratio SiO₂/Al₂O₃=20), prepared in accordancewith U.S. Pat. No. 3,308,069, pelleted and granulated within a range of20-40 mesh, are charged into the isothermal zone of a fixed bed reactor,with quartz above and below as inert filler. The reactor is heated to200° C. for at least 2 hours, under a stream of nitrogen againstatmospheric pressure. Maintaining under a stream of inert gas, thereactor is cooled to room temperature, and the reagents are then feduntil the reactor is pressurized to 40 bars. The mixture of reagents fedconsists of 1,2,4-trimethylbenzene, naphthalene, methanol in thefollowing molar ratios: 1,2,4-trimethylbenzene/naphthalene=10,methanol/naphthalene=3. At this point the mixture is heated and broughtto a temperature of 350° C. In this test the conditions are thereforeliquid phase, with respect to the state of reagents and products. TheWHSV (hours⁻¹) with respect to the total mixture is 0.86. The productsleaving the reactor are cooled and analyzed by gaschromatography.Samples are taken of the products at regular time intervals (time onstream). The conversion of the methanol is always total.

The conversion of naphthalene after 95 hours is equal to 26.1%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (moles): 9.9

selectivity 2,6-dimethylnaphthalene (% moles): 2.5

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 4.8

selectivity methylnaphthalenes (% moles): 77.5

selectivity polymethylnaphthalenes (% moles): 12.6

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 25.6

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 48.9

molar ratio 2,6/2,7 dimethylnaphthalene: 1.3

From the results indicated above, it can be observed that beta zeolite,under the same conditions as zeolite ZSM-12, is less active, lessselective to dimethylnaphthalenes, less selective to 2,6 isomer and2,6-+1,6-+1,5-isomers, on the contrary beta zeolite is more selectivewith respect to the formation of the undesired products tri- andtetramethylnaphthalenes and is less favourable towards the formation of2,6 isomer with respect to the 2,7 isomer.

EXAMPLE 6 (comparative)

2.3 gr of zeolite ZSM-5 (MFI) (PQ CBU3070E, molar ratio SiO₂/Al₂O₃=30),in acid form, pelleted and granulated within a range of 20-40 mesh, arecharged into the isothermal zone of a fixed bed reactor, with a quartzabove and below as inert filler. The reactor is heated to 200° C. for atleast 2 hours, under a stream of nitrogen against atmospheric pressure.Maintaining under a stream of inert gas, the reactor is cooled to roomtemperature, and the reagents are then fed until the reactor ispressurized to 40 bars. The mixture of reagents fed consists of1,2,4-trimethylbenzene, naphthalene, methanol in the following molarratios: 1,2,4-trimethylbenzene/naphthalene=10, methanol/naphthalene=3.At this point the mixture is heated and brought to a temperature of 450°C. In this test the conditions are therefore gas phase. The WHSV(hours⁻¹) with respect to the total mixture is 0.75. The productsleaving the reactor are cooled and analyzed by gaschromatography.Samples are taken of the products at regular time intervals (time onstream). The conversion of the methanol is always total.

a) The conversion of naphthalene after 96 hours is equal to 21.8%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 7.8

selectivity 2,6-dimethylnaphthalene (% moles): 3.6

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 3.6

selectivity methylnaphthalenes (% moles): 78.3

selectivity polymethylnaphthalenes (% moles): 0.0

selectivity to ethylnaphthalenes (% moles): 14.0

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 26.3

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 46.3

molar ratio 2,6/2,7 dimethylnaphthalene: 0.9

From the results indicated above, it can be observed that zeolite ZSM-5is less active than ZSM-12. In fact, to have comparable conversions theZSM-5 must be used at much higher temperatures (450° C. against 350° C.)and under gas phase conditions. As well as being less active, zeoliteZSM-5 is more rapidly deactivated, with a considerable formation ofcarbonaceous residues, it is less selective to dimethylnaphthalenes,less selective to 2,6 isomer and to 2,6-+1,6-+1,5-isomers. ZSM-5 alsoproduces a significant quantity of undesired ethylnaphthalenes, whereasthe molar ratio between the 2,6 and 2,7 isomers of dimethylnaphthaleneis practically the thermodynamic ratio and it is not unbalanced towardsthe 2,6 isomer, which is the isomer of particular interest.

EXAMPLE 7 (comparative)

2 gr of zeolite ZSM-12 (MTW), prepared in accordance with example 1,pelleted and granulated within a range of 20-40 mesh, are charged intothe isothermal zone of a fixed bed reactor, with quartz above and belowas inert filler. The reactor is heated to 200° C. for at least 2 hours,under a stream of nitrogen against atmospheric pressure. Maintainingunder a stream of inert gas, the reactor is cooled to room temperature,and the reagents are then fed until the reactor is pressurized to 30bars. The mixture of reagents fed consists of 1,2,4-trimethylbenzene,naphthalene, methanol in the following molar ratios:1,2,4-trimethylbenzene/naphthalene=10, methanol/naphthalene=3. At thispoint the mixture is heated and brought to a temperature of 400° C. Inthis test the conditions are therefore gas phase. The WHSV (hours⁻¹)with respect to the total mixture is 0.86. The products leaving thereactor are cooled and analyzed by gaschromatography. Samples are takenof the products at regular time intervals (time on stream). Theconversion of the methanol is always total.

a) The conversion of naphthalene after 28.5 hours is equal to 76.7%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 42.5

selectivity 2,6-dimethylnaphthalene (% moles): 10.5

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 22.1

selectivity methylnaphthalenes moles): 44.0

selectivity polymethylnaphthalenes (% moles): 13.6

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 24.6 (it should be remembered that the thermodynamic isomericdistribution estimates 12.0% of 2,6 (PU))

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 52.1 (thermodynamic 32%)

molar ratio 2,6/2,7 dimethylnaphthalene: 1.5 (thermodynamic 1)

The test is continued and the conversion of naphthalene after 95 hoursis equal to 21.0%

The selectivities with respect to naphthalene are:

selectivity dimethylnaphthalenes (% moles): 20.1

selectivity 2,6-dimethylnaphthalene (% moles): 4.1

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 8.2

selectivity methylnaphthalenes (% moles): 78.3

selectivity polymethylnaphthalenes (% moles): 1.7

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 20.6 (it should be remembered that the thermodynamic isomericdistribution estimates 12.0% of 2,6 (PU))

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 40.7 (thermodynamic 32%)

molar ratio 2,6/2,7 dimethylnaphthalene: 1.2 (thermodynamic 1)

It is further demonstrated that operating in gas phase provides worseperformances both in terms of the deactivation rate of the catalyst andselectivity to 2,6 isomer and 2,6-+1,6-+1,5 isomers. The formation of2,6 isomer with respect to 2,7 isomer is also less favourable.

EXAMPLE 8 (comparative)

4 gr of zeolite ZSM-12 (molar ratio SiO₂/Al₂O₃=100), prepared inaccordance with example 1, pelleted and granulated within a range of20-40 mesh, are charged into the isothermal zone of a fixed bed reactor,with quartz above and below as inert filler. The reactor is heated to200° C. for at least 2 hours, under a stream of nitrogen againstatmospheric pressure. Maintaining under a stream of inert gas, thereactor is heated to a temperature of 320° C., and the reagents are thenfed into the reactor at atmospheric pressure. The reaction thereforetakes place under gas phase conditions. The mixture of reagents fedconsists of methylnaphthalenes and methanol in the following ratios:2-methylnaphthalene/1-methylnaphthalene=1.5;methanol/methylnaphthalenes=0.1.

The WHSV (hours⁻¹) with respect to the total mixture is 0.20. Theproducts leaving the reactor are cooled and analyzed bygaschromatography. Samples are taken of the products at regular timeintervals (time on stream).

The conversion of the methanol is always total.

a) The conversion of methylnaphthalene after 3 hours is equal to 13.56%

The selectivities with respect to converted methylnaphthalene are:

selectivity dimethylnaphthalenes (% moles): 70.0

selectivity 2,6-dimethylnaphthalene (% moles): 7.3

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 19.7

selectivity naphthalene (% moles): 11.2

selectivity trimethylnaphthalenes (% moles): 13.5

selectivity heavy products (% moles) 5.3

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 10.5

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 28.1

molar ratio 2,6/2,7 dimethylnaphthalene: 1.2

The test is continued and the conversion of methylnaphthalene after 24hours is equal to 3.0%

The selectivities with respect to methylnaphthalene are:

selectivity dimethylnaphthalenes (% moles): 94.6

selectivity 2,6-dimethylnaphthalene (% moles): 9.2

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 24.4

selectivity methylnaphthalenes (% moles): 1.9

selectivity trimethylnaphthalenes (% moles): 3.2

selectivity heavy products (% moles): 0.3

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 9.7

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 25.8

molar ratio 2,6/2,7 dimethylnaphthalene: 1.3 (thermodynamic 1)

It is evident that the preparation of 2,6-dimethylnaphthalene under theconditions of the prior art, i.e. under gas phase conditions and withouta solvent, causes a precocious and significant deactivation of thecatalyst.

The selectivity to total dimethylnaphthalenes is high, but only whencoinciding with very low conversions of methylnaphthalene and with adeactivated catalyst.

The distribution of the isomers among the dimethylnaphthalenes isdifferent from that obtained when operating according to the process ofthe present invention: in particular the molar ratio between the 2,6isomer and total dimethylnaphthalenes is much lower (10 against 30-35),as is also that between 2,6+1,6 +1,5 and the total dimethylnaphthalenes(28 against 60-70). In addition there is the formation ofnon-recoverable heavy products, such as dinaphthylmethanes possibly withfrom 1 to 3 methyl groups bound to the aromatic groups.

EXAMPLE 9

4 gr of zeolite ZSM-12 (molar ratio SiO₂/Al₂O₃ is 100), prepared inaccordance with example 1, pelleted and granulated within a range of20-40 mesh, are charged into the isothermal zone of a fixed bed reactor,with quartz above and below as inert filler. The reactor is heated to200° C. for at least 2 hours, under a stream of nitrogen againstatmospheric pressure. Maintaining under a stream of inert gas, thereactor is cooled to room temperature, and the reagents are then feduntil the reactor is pressurized to 35 bars. The mixture of reagents fedconsists of 1,2,4-trimethylbenzene, naphthalene, 1-methylnaphthalene,methylnaphthalene, methanol in the following molar ratios:1,2,4-trimethylbenzene/naphthalene groups=18, methanol/naphthalene=2,methanol/methylnaphthalene=1. Naphthalene groups refer to the sum inmoles of naphthalene, 1-methylnaphthalene, 2-methylnaphthalene 30% ofthe naphthalene groups consists of naphthalene, the remaining 70%consists of 30% of 1-methylnaphthalene and 70% of 2-methylnaphthalene.At this point the mixture is heated and brought to a temperature of 300°C. In this test the conditions are therefore liquid phase, with respectto the state of reagents and products. The WHSV (hours⁻¹) with respectto the total mixture is 0.43. The products leaving the reactor arecooled and analyzed by gaschromatography. Samples are taken of theproducts at regular time intervals (time on stream). The conversion ofthe methanol is always total.

The selectivities calculated with respect to the naphthalene groups asif they all derived from simple naphthalene are:

selectivity dimethylnaphthalenes (% moles): 40.3

selectivity 2,6-dimethylnaphthalene (% moles): 13.3

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 27.4

selectivity methylnaphthalenes moles): 56.2

selectivity polymethylnaphthalenes (% moles): 3.4

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 33.3

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 67.9

molar ratio 2,6/2,7 dimethylnaphthalene: 2.5

The mole % of dimethylnaphthalenes out of the total naphthalene groupsis 33.2.

Under the same experimental conditions and with the same t.o.s., i.e.the deactivation of the catalyst, a mixture is reacted which differsfrom the previous one in that it contains naphthalene alone asnaphthalene substrate. The following selectivities with respect tonaphthalene correspond to a conversion of 32.9%:

selectivity dimethylnaphthalenes (% moles): 20.9

selectivity 2,6-dimethylnaphthalene (% moles): 7.1

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 13.6

selectivity methylnaphthalenes (% moles): 79.1

selectivity polymethylnaphthalenes (% moles): 0.0

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 34.2

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 65.2

molar ratio 2,6/2,7 dimethylnaphthalene: 2.6

On comparing the two tests, it can be observed that at a certaintemperature, it is possible to increase the yield todimethylnaphthalenes substituting part of the naphthalene withmethylnaphthalenes.

EXAMPLE 10

Two tests are carried out under the following conditions:

4 gr of zeolite ZSM-12 (molar ratio SiO₂/Al₂O₃=100), prepared inaccordance with example 1, pelleted and granulated within a range of20-40 mesh, are charged into the isothermal zone of a fixed bed reactor,with quartz above and below as inert filler. The reactor is heated to200° C. for at least 2 hours, under a stream of nitrogen againstatmospheric pressure. Maintaining under a stream of inert gas, thereactor is cooled to room temperature, and the reagents are then feduntil the reactor is pressurized to 35 bars.

In the first test, the naphthalene hydrocarbon used is only naphthaleneand the reagent mixture fed consists of 1,2,4-trimethylbenzene,naphthalene and methanol in the following molar ratios:1,2,4-trimethylbenzene/naphthalene=18, methanol/naphthalene=2.

In the second test, the naphthalene hydrocarbon fed consists of 73.7% ofnaphthalene and the remaining 26.3% of dimethylnaphthalenes and thereagent mixture used consists of 1,2,4-trimethylbenzene, naphthalene,dimethylnaphthalenes and methanol. The methanol is chargedproportionally to the naphthalene groups, according to the followingcriterium: 2 moles of methanol per mole of naphthalene, no mole ofmethanol per mole of dimethylnaphthalene. The molar ratio1,2,4-trimethylbenzene/naphthalene groups is equal to 18. Naphthalenegroups refer in this case to the sum in moles of naphthalene anddimethylnaphthalenes. The operation is carried out at a temperature of320° C., under liquid phase conditions, at a total WHSV of 0.43 hours⁻¹.

The two tests are carried out so that the equivalent conversion iscomparable, the equivalent conversion (% mole) referring to the ratio,multiplied by 100, between the sum of moles of methylnaphthalenes,dimethylnaphthalenes, trimethylnaphthalenes, and polymethylnaphthalenesamong the products and the total moles of naphthalenes among theproducts, comprising therefore also the naphthalene.

The equivalent conversion is then calculated as if all the naphthalenegroups fed, methylated and non-methylated, are simple naphthalenegroups.

In the first test the following results are obtained:

conversion=78.4%

composition of the products % mol (after removal of the1,2,4-trimethylbenzene):

naphthalene 21.6 methylnaphthalenes 27.4 dimethylnaphthalenes 36trimethylnaphthalenes 15

In the second test the results obtained are the following:

conversion=80.8%

composition of the products % mol (without the benzene hydrocarbon):

naphthalene 19.2 methylnaphthalenes 25.3 dimethylnaphthalenes 39.2trimethylnaphthalenes 16.4

Comparison of the data indicated in the previous tables demonstratesthat the distribution of the main products in both cases is very similarand consequently the dimethylnaphthalenes fed entered the reaction cyclewithout simple accumulation, i.e. inert behaviour, or only additionalalkylation.

These data are in accordance with a process in which contemporaneoustransalkylation, deproportioning, alkylation and isomerization reactionstake place.

Table A below indicates the distribution in molar % of the isomers ofdimethylnaphthalene resulting from the first test, in which thesubstrate used was exclusively naphthalene:

TABLE A isomer mol % 2,6 28.6 2,7 16 1,3 + 1,7 17.5 1,6 27.2 1,4 + 2,33.1 1,5 4.7 1,2 3 1,8 0

Table B below indicates the distribution in molar % of the isomers ofdimethylnaphthalene resulting from the second test, in which thesubstrate used consisted of 73.7% of naphthalene and 26.3% ofdimethylnaphthalenes (the table also indicates the percentagedistribution of the isomers in the dimethylnaphthalene fed)

TABLE B isomer feeding mol % products mol % 2,6 — 22.6 2,7 17.2 13.71,3 + 1,7 — 26 1,6 — 21 1,4 + 2,3 82.8 10.6 1,5 — 3.3 1,2 — 2.8 1,8 — 0

Comparison between the tables shows that the 2,3-dimethylnaphthalenefully entered the reactive system; the same can be said for the 2,7isomer, as there is no registration of its accumulation.

Table C below summarizes the molar % composition of the productsrelating to the two tests discussed above: the first and second columnsrefer respectively to the feeding of naphthalene alone as naphthalenesubstrate and to the composition in molar % of products and isomersobtained in the first test; the third and fourth columns referrespectively to the composition of the naphthalene substrate and thecomposition in molar % of products and isomers obtained in the secondtest.

TABLE C (composition in % moles) Feeding Products Feeding ProductsNaphthalene 100 21.6 73.7 19.2 1-methylnaphthalene — 8.5 — 7.72-methylnaphthalene — 18.9 — 17.6 Methylnaphthalenes — 27.4 — 25.3Dimethylnaphthalenes — 36 26.3 39.2 2,6 isomer — 10.3 — 8.9 2,7 isomer —5.8 4.3 5.4 1,3 + 1,7 isomers — 6.3 — 10.2 1,6 isomer — 9.8 — 8.2 1,4 +2,3 isomers — 1.1 21.8 4.1 1,5 isomer — 1.7 — 1.3 1,2 isomer — 1.1 — 1.11,8 isomer — 0 — 0 Trimethylnaphthalenes 15 — 16.4

Also in this table it can be seen that 2,3-dimethylnaphthalene fullyenters the reactive system; the same can be said for the 2,7 isomer, asthere is no registration of its accumulation.

EXAMPLE 11

Using the same procedure as the previous example, the followingexperimentations are carried out at 300° C. and 35 bars. The total WHSVis 0.43 hours⁻¹. In the first test the naphthalene hydrocarbon used isnaphthalene alone and the reagent mixture fed consists of1,2,4-trimethylbenzene, naphthalene and methanol in the following molarratios: 1,2,4-trimethylbenzene/naphthalene=18, methanol/naphthalene=2.

The results obtained, expressed as composition in moles, are thefollowing:

Equivalent conv. % 67.10 Naphthalene 32.90 1-methylnaphthalene 10.502-methylnaphthalene 24.20 Methylnaphthalenes 34.80 Dimethylnaphthalenes29.20 2,6 isomer 9.40 2,7 isomer 4.00 1,3 + 1,7 isomers 4.60 1,6 isomer8.50 1,4 + 2,3 isomers 0.80 1,5 isomer 1.30 1,2 isomer 0.50 1,8 isomer0.00 Trimethylnaphthalenes 3.10

In the second test a feeding is used which repeats the spectrum ofproducts obtained from the first experimentation, except that:

the 2,6-dimethylnaphthalene, 1,6-dimethylnaphthalene and1,5-dimethylnaphthalene (belonging to the same isomerization group asdefined in EP 519165) are replaced by an equal quantity in moles ofnaphthalene;

the 1,3, 2,3 and 1,4 isomers (as they belong to the same isomerizationgroup as defined in EP 519165) are replaced by the 2,3 isomer;

the 1,7 isomer, which is not anlaytically separated from the 1,3 isomer,and cannot therefore be quantified, is substituted with the 2,3 isomer.

This second test therefore simulates a recycling of products and isomerswhich are different from the desired product in the synthesis reactor of2,6-dimethylnaphthalene. The composition in % moles of the naphthalenesubstrate used in the second test is therefore the following:

Naphthalene 53.70 1-methylnaphthalene 10.60 2-methylnaphthalene 24.50Methylnaphthalenes 35.10 Dimethylnaphthalenes 11.20 2,6 isomer 0.00 2,7isomer 4.90 1,3 + 1,7 isomers 0.00 1,6 isomer 0.00 1,4 + 2,3 isomers5.70 1,5 isomer 0.00 1,2 isomer 0.50 1,8 isomer 0.00Trimethylnaphthalenes 0.00

The reagent mixture fed also contains 1,2,4-trimethylbenzene andmethanol. The methanol is charged proportionally to the naphthalenegroups, according to the following criterium: two moles of methanol permole of naphthalene, one mole of methanol per mole of methylnaphthalene,no mole of methanol per mole of dimethylnaphthalene. The molar ratio1,2,4-trimethylbenzene/naphthalene groups is equal to 18. Naphthalenegroups refer to the sum in moles of naphthalene, methylnaphthalenes anddimethylnaphthalenes.

The results obtained in this second experimentation, expressed ascomposition in % moles, are indicated in Table D below, first column,where they are compared with the spectrum of products obtained startingfrom naphthalene alone as naphthalene substrate (second column), withthe same equivalent conversion.

TABLE D Equivalent conv. % 89.00 89.30 Naphthalene 11.00 10.701-methylnaphthalene 7.50 7.30 2-methylnaphthalene 17.60 16.60Methylnaphthalenes 25.10 23.90 Dimethylnaphthalenes 49.80 50.20 2,6isomer 15.00 15.20 2,7 isomer 8.10 8.40 1,3 + 1,7 isomers 8.50 8.40 1,6isomer 13.60 14.10 1,4 + 2,3 isomers 1.50 1.10 1,5 isomer 2.20 2.30 1,2isomer 1.10 0.70 1,8 isomer 0.00 0.00 Trimethylnaphthalenes 14.10 15.20

As can be seen from the above table, from the test which simulates therecycling, there is a spectrum of products (column 1) which ispractically undistinguishable from that obtained, with the sameequivalent conversion, starting from naphthalene alone (column 2): thesedata show that the dimethylnaphthalenes fed have entered the reactioncycle, as there is no simple accumulation, i.e. inert behaviour, or onlyadditional alkylation, and they are also in accordance with a process inwhich transalkylation, deproportioning, alkylation and isomerizationreactions contemporaneously contribute to the formation of the desiredproduct 2,6-dimethylnaphthalene.

EXAMPLE 12

Using the same procedure as the previous example, a test is carried outat 300° C. and 35 bars. The total WHSV is 0.43 hours⁻¹.Methylnaphthalene is used as naphthalene hydrocarbon, consisting of 70%of 2-methylnaphthalene and 30% of 1-methylnaphthalene. The reagentmixture fed consists of 1,2,4-trimethylbenzene, methylnaphthalene andmethanol in the following molar ratios:1,2,4-trimethylbenzene/methylnaphthalene=10,methanol/methylnaphthalene=0.5. The methylnaphthalene fed consists of70% of 2-methylnaphthalene and 30% of 1-methylnaphthalene.

During the test two samplings are taken, with relative GC analysis ofthe products, and the results are indicated in the following table:

time on stream time on steam (hours) (hours) Feeding 92 130 — Conv. MN %65.9 64.4 — Select. DMNi (mol %) 85.2 86.5 Select. 2,6 DMN (mol %) 29.630.2 Select. 2,6.1,6-1,5 DMN (mol %) 60 61.2 Select. N (mol %) 1.8 1.8Select. PMNi (mol %) 12.9 11.7 2,6-DMN/total DMNi (mol %) 34.7 34.92,6-1,6-1,5-DMN/total DMNi (mol %) 70.4 70.7 Molar ratio 2,6/2,7-DMN 2.52.6 Composition in weight % (solvent free) N 1 1 1-MN 9.7 10.2 30 2-MN22.1 23.1 70 MNi 31.9 33.3 100  EtNi 0 0 DMNi 57.6 57.3 2,6-DMN 20 202,7-DMN 8 7.8 1,3-1,7-DMN 7.6 7.4 1,6-DMN 17.7 17.7 1,4-2,3-DMN 0.6 0.61,5-DMN 2.9 2.8 1,2-DMN 0.8 1 1,8-DMN 0 0 TMNi 9.5 8.4

In the above table N=naphthalene; MN=methylnaphthalene;MNi=methylnaphthalenes; EtNi=ethylnaphthalenes; DMN=dimethylnaphthalene;DMNi=dimethylnaphthalenes; TMNi=trimethylnaphthalenes;PMNi=polymethylnaphthalenes.

Under these conditions there is a very high selectivity todimethylnaphthalenes, whereas the distribution of the isomers internallydoes not differ from the previous examples. The high total selectivityto total DMNi, is reflected in a greater weight % production of 2,6-DMNthan the previous examples.

EXAMPLE 13 (comparative)

Two grams of Y zeolite (FAU) (TOSOH HSZ 330 HUA, molar ratioSiO₂/Al₂O₃=6), in acid form, pelleted and granulated within a range of20-40 mesh, are charged into the isothermal zone of a fixed bed reactor,with quartz above and below as inert filler. The temperature of thereactor is brought to 200° C. for at least 2 hours, under a stream ofnitrogen against atmospheric pressure. Maintaining under a stream ofinert gas, the reactor is cooled to room temperature, and the reagentsare then fed until the reactor is pressurized to 40 bars. The mixture ofreagents fed consists of 1,2,4-trimethylbenzene, naphthalene, methanolin the following molar ratios: 1,2,4-trimethylbenzene/naphthalene=10,methanol/naphthalene=3. At this point the mixture is heated and broughtto a temperature of 350° C. Therefore, in this test we are in theconditions of liquid phase, with respect to the state of reagents andproducts. The WHSV (h⁻¹) (with respect to the total mixture) is 0.86.The products at the output the reactor are cooled and analyzed bygaschromatography. Samples are taken of the products at regular time onstream intervals.

The conversion of the methanol is always total.

The conversion of the naphthalene after 95 h is 21.7%.

The selectivities with respect to the naphthalene are:

selectivity dimethylnaphthalenes (% moles): 14.8

selectivity 2,6-dimethylnaphthalene (% moles): 1.4

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 3.9

selectivity methylnaphthalenes (% moles): 84.1

selectivity polymethylnaphthalenes (% moles): 1.1

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 9.7

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 26.5

molar ratio 2,6/2,7 dimethylnaphthalene: 1.1

From the results indicated above it can be observed that Y zeolite, withrespect to ZSM-12 zeolite, under the same conditions, is less active,less selective to dimethylnaphthalenes, less selective to the 2,6 isomerand to the 2,6-+1,6-+1,5- isomers. In addition it is much less selectivetowards the formation of the 2,6 isomer with respect to the 2,7 isomer(ratio close to the thermodynamic ratio) and much less selective towardsthe formation of the 2,6 isomer with respect to the other isomers ofdimethylnaphthalenes.

EXAMPLE 14 (comparative)

Two grams of ZSM5 zeolite (MFI) (PQ CBU3070E, ratio SiO₂/Al₂O₃=30), inacid form, pelleted and granulated within a range of 20-40 mesh, arecharged into the isothermal zone of a fixed bed reactor, with quartzabove and below as inert filler. The temperature of the reactor isbrought to 200° C. for at least 2 hours, under a stream of nitrogenagainst atmospheric pressure. Maintaining under a stream of inert gas,the reactor is cooled to room temperature, and the reagents are then feduntil the reactor is pressurized to 40 bars. The mixture of reagents fedconsists of 1,2,4-trimethylbenzene, naphthalene, methanol in thefollowing molar ratios: 1,2,4-trimethylbenzene/naphthalene=10,methanol/naphthalene=3. At this point the mixture is heated and broughtto a temperature of 350° C. Therefore in this test the conditions are ofliquid phase, with respect to the state of reagents and products. TheWHSV (h⁻¹) (with respect to the total mixture) is 0.86. The products atthe output of the reactor are cooled and analyzed by gaschromatography.Samples are taken of the products at regular time on stream intervals.

The conversion of the methanol is always total.

The conversion of the naphthalene after 49 h is 6.7%.

The selectivities with respect to the naphthalene are:

selectivity dimethylnaphthalenes (% moles): 24.5

selectivity 2,6-dimethylnaphthalene (% moles): 6.6

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 6.6

selectivity methylnaphthalenes (% moles): 57.3

selectivity to ethylnaphthalenes (% moles): 18.2

selectivity polymethylnaphthalenes (% moles): 0.0

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 27.0

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 27.0

molar ratio 2,6/2,7 dimethylnaphthalene: 0.5

EXAMPLE 15

The experimental data obtained with ZSM-12 zeolite (prepared accordingto example 1) operating under the same conditions as example 2 and theprevious comparative example, after 49 hours of time on stream, areprovided hereunder.

Conversion of naphthalene: 75.8%.

The selectivities with respect to the naphthalene are:

selectivity dimethylnaphthalenes (% moles): 47.9

selectivity 2,6-dimethylnaphthalene (% moles): 13.8

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 29.2

selectivity methylnaphthalenes (% moles): 44.3

selectivity polymethylnaphthalenes (% moles): 7.8

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 28.9

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 61.0

molar ratio 2,6/2,7 dimethylnaphthalene: 1.9

On comparing the above data with those of the previous example 14, itcan be observed that ZSM-5 zeolite, with respect to ZSM-12 zeolite,under the same conditions, is much less active, less selective todimethylnaphthalenes, less selective to 2,6 isomer, less selective to2,6-+1,6-+1,5-isomers. In addition it produces a large quantity ofethylnaphthalenes, which are absent in the spectrum of ZSM-12 products.The molar ratio between 2,6 and 2,7 isomers is far from thethermodynamic ratio and is decisively unbalanced towards 2,7.

EXAMPLE 16 (comparative)

Two grams of Y zeolite (FAU) (TOSOH HSZ 330 HUA, molar ratioSiO_(2/)Al₂O₃=6), in acid form, pelleted and granulated within a rangeof 20-40 mesh, are charged into the isothermal zone of a fixed bedreactor, with quartz above and below as inert filler. The temperature ofthe reactor is brought to 200° C. for at least two hours, under a streamof nitrogen against atmospheric pressure. Maintaining under a stream ofinert gas, the reactor is cooled to room temperature, and the reagentsare then fed until the reactor is pressurized to 40 bars. The mixture ofreagents fed consists of 1,2,4-trimethylbenzene and naphthalene. Themolar ratio 1,2,4-trimethylbenzene/naphthalene=10. In this examplemethanol is not present in the reaction mixture. After pressurizing thereactor, it is heated and brought to a temperature of 350° C. Therefore,in this test the conditions are of liquid phase, with respect to thestate of reagents and products. The WHSV (h⁻¹) (with respect to thetotal mixture) is 0.86. The products at the output of the reactor arecooled and analyzed by gaschromatography. Samples are taken of theproducts at regular time on stream intervals.

The conversion of the naphthalene after 95 h is 22.9%.

The selectivities with respect to the naphthalene are:

selectivity dimethylnaphthalenes (% moles): 15.6

selectivity 2,6-dimethylnaphthalene (% moles): 1.7

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 4.4

selectivity methylnaphthalenes (% moles): 83.1

selectivity polymethylnaphthalenes (% moles): 1.3

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 10.8

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 28.0

molar ratio 2,6/2,7 dimethylnaphthalene: 1.0

EXAMPLE 17

Two grams of ZSM12 zeolite (MTW) (prepared as described in example 1)pelleted and granulated within a range of 20-40 mesh, are charged intothe isothermal zone of a fixed bed reactor, with quartz above and belowas inert filler. The temperature of the reactor is brought to 200° C.for at least 2 hours, under a stream of nitrogen against atmosphericpressure. Maintaining under a stream of inert gas, the reactor is cooledto room temperature, and the reagents are then fed until the reactor ispressurized to 40 bars. The mixture of reagents fed consists of1,2,4-trimethyl-benzene and naphthalene. The molar ratio1,2,4-trimethylbenzene/naphthalene=10. In this example methanol is notpresent in the reaction mixture. After pressurizing the reactor, it isheated and brought to a temperature of 350° C. Therefore in this testthe conditions are of liquid phase, with respect to the state ofreagents and products. The WHSV (h⁻¹) (with respect to the totalmixture) is 0.86. The products at the output of the reactor are cooledand analyzed by gaschromatography. Samples are taken of the products atregular time on stream intervals.

The conversion of the naphthalene after 95 h is 49.5%.

The selectivities with respect to the naphthalene are:

selectivity dimethylnaphthalenes (% moles): 32.5

selectivity 2,6-dimethylnaphthalene (% moles): 9.0

selectivity 2,6/1,6/1,5-dimethylnaphthalene (% moles): 19.0

selectivity methylnaphthalenes (% moles): 64.8

selectivity polymethylnaphthalenes (% moles): 2.7

molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes (%moles): 27.7

molar ratio 2,6-1,6-1,5-dimethylnaphthalene/total dimethylnaphthalenes(% moles): 57.6

molar ratio 2,6/2,7 dimethylnaphthalene: 2.0

On comparing the data of example 17 above with the data of the previouscomparative example 16, both carried out without methanol, it can beobserved that Y zeolite, with respect to ZSM-12 zeolite, under the sameconditions, is less active, less selective to dimethylnaphthalenes, lessselective to 2,6 isomer and to 2,6-+1,6-+1,5- isomers. In addition it ismuch less selective towards the formation of 2,6 isomer with respect to2,7 isomer (ratio close to the thermodynamic ratio) and much lessselective towards the formation of 2,6 isomer with respect to the otherisomers of dimethylnaphthalenes.

EXAMPLE 18 (comparative)

Two grams of ZSM12 zeolite (MTW) (prepared as described in example 1),pelleted and granulated within a range of 20-40 mesh, are charged intothe isothermal zone of a fixed bed reactor, with quartz above and belowas inert filler. The temperature of the reactor is brought to 200° C.for at least two hours, under a stream of nitrogen against atmosphericpressure. Maintaining under a stream of inert gas, the reactor is cooledto room temperature, and the reagents are then fed until the reactor ispressurized to 40 bars. The mixture of reagents fed consists of1-methylnaphthalene (liquid at room temperature and also at 350° C., 40bars) and methanol. The molar ratio methanol/1-methylnaphthalene isequal to 0.5. After pressurizing the reactor, it is heated and broughtto a temperature of 350° C. The WHSV (h⁻¹) (with respect to the totalmixture) is 0.75. The products at the output of the reactor are cooledand analyzed by gaschromatography. Samples are taken of the products atdifferent time on stream intervals. The conversion of the methanol isalways total. The conversion of 1-methylnaphthalene, after 20 hours oftime on stream is equal to 16.8%.

The molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes is7.7

The molar ratio 2,6-1,6-1,5-dimethylnaphthalene/totaldimethylnaphthalenes is 25.7

The molar ratio 2,6/2,7 dimethylnaphthalene is 0.9

The conversion of 1-methylnaphthalene, after 23 hours of time on streamis equal to 12.0%.

The molar ratio 2,6-dimethylnaphthalene/total dimethylnaphthalenes is6.1

The molar ratio 2,6-1,6-1,5-dimethylnaphthalene/totaldimethylnaphthalenes is 27.1

The molar ratio 2,6/2,7 dimethylnaphthalene is 1.0

What is claimed is:
 1. A process for preparing 2,6-dimethylnaphthalenewhich comprises subjecting 1,6 and/or 1,5-dimethylnaphthalenes,optionally mixed with other isomers of dimethylnaphthalene, toisomerization, which comprises treating said compounds at a temperatureranging from 100 to 400° C., under at least partially liquid phaseconditions, in the presence of an MTW zeolite, thereby producing aproduct comprising 2,6-dimethylnaphtalene.
 2. The process according toclaim 1, wherein the temperature ranges from 120 to 250° C.